The temperature dependence of the ionic product of water (pK(w)) is investigated theoretically by means of ab initio electronic structure theory combined with the extended reference interaction site method in statistical mechanics of molecular liquids (RISM-SCF/MCSCF method). The chemical equilibrium H2O + H2O reversible arrow H3O+ + OH- is studied, in which water molecules? hydronium ions, and hydroxide ions are regarded as "solute" molecules in aqueous solution. Molecular geometries, electronic structures, pair correlation functions, and free energy components of those species as well as their temperature dependence are calculated. It is shown that the hydroxide anion is polarized more easily by surrounding solvent compared to the other species. The solvent-induced electronic structure relaxes toward that in the gas phase as temperature increases. The hydroxide anion exhibits the largest temperature dependence in the electronic structure as well as in solvation structure. It is found that changes in the solvation free energies drive the chemical equilibrium toward the left-hand side (association) as temperature increases, while energies associated with solvent-induced reorganization of electronic structure make the opposite contribution. The temperature dependence of pK(w). is dominated by the latter contribution, which gives rise to good agreement with the experimental results. It is suggested that the observed temperature dependence of pK(w) is related to the great sensitivity of the electronic structure of OH- on the solvent effect.